The present invention relates to a method and an apparatus for effecting by means of microwaves a substantially uniform heating of a material attached in a cavity. The invention is intended to be used especially at the heating of plastic material of both thermosetting resin and thermoplastic resin type.
Heating by microwaves is known since a long time ago. It was not clearly known, however, how a material attached in a cavity could be heated uniformly, i.e. how a uniform distribution of energy in the cavity could be obtained.
The present invention provides a method and an apparatus for effecting a substantially uniform energy distribution in a cavity when the dielectric losses of the material attached in the cavity are not too great. Examples of such dielectric materials, for which the invention can be utilized, are plastics, glass, paper, wood, most of the ceramic materials and many rubber types. The invention can be applied also to other dielectric materials with dielectric properties similar to those in the example. At the injection moulding of thermoplastic resin, liquid plastic of a relatively high temperature is injected into a relatively cold mould. During the injection operation the plastic is cooled, and thereby its viscosity increases substantially. As a result thereof, the mould at times is not filled entirely, or the temperature of the plastic being injected is increased, thereby deteriorating the quality. For filling the mould entirely, the injection pressure must be increased, which gives rise to internal stresses in the detail. A further disadvantage is that the mould must be dimensioned for a higher pressure. It is, therefore, desired to supply heat during the injection operation and thereafter to rapidly cool the details. This has been rendered possible by the present invention.
At the injection moulding of thermosetting resin, today the low production capacity is one of the most serious reasons restricting a wider use of this technique. By microwave heating, the setting times can be reduced substantially.
There exist various possibilities according to the present invention to bring about a uniform heating, i.e. an electic field energy density in the cavity which is as uniform as possible. For a better understanding of the invention, its method and apparatus are described in the following. The characterizing features of the invention become apparent from the attached claims.